As an auxiliary storage device mounted on a computer, an auxiliary storage device including a hard disk drive (HDD) (hereinafter, “HDD device”) is used (see Japanese Patent Application Laid-Open NO. 2004-30837).
In recent years, an auxiliary storage device including, as a recording medium, a nonvolatile semiconductor memory such as a NAND flash memory (so-called solid state disk, hereinafter, “SSD device”) is mounted on a computer instead of the HDD device. In the SSD device, a plurality of NAND flash memories (hereinafter, “NANDs”) and a controller integrated circuit (IC) that controls the NANDs are mounted on a printed circuit board via electrodes (bumps). The SSD device is mounted on the computer after being housed in a housing generally having an external dimension and a shape same as those of the HDD device specified by a standard (e.g., a housing having a size and a shape same as those of a 2.5-inch HDD device). There are several kinds of standards concerning the housing of the HDD device according to the sizes of magnetic disks. In general, a housing such as that for the 2.5-inch HDD device is a box made of metal.
When the SSD device actually operates (when data is actually read and written), a controller generates heat because switching operation is repeatedly performed at high speed. A part of the heat generated from the controller is transmitted to the printed circuit board via the bump set in contact with the controller and further transmitted to the NANDs via a wiring pattern on the printed circuit board the bumps set in contact with the NANDs. In an operation principle, under a high-temperature environment, the NANDs tend to unsteady operate because leak current increases. Therefore, operation guarantee temperature for the NANDs is set low compared with other kinds of ICs. For example, whereas the operation guarantee temperature of the NANDs is about 85° C., the operation guarantee temperature of the other kinds of ICs is about 100° C.
Therefore, in the SSD device, it is necessary to cool the controller to suppress the transmission of the heat from the controller to the NANDs. In the case of the HDD device including a magnetic disk as a recording medium, because the controller and the magnetic disk are arranged separately from each other, the heat generated from the controller does not affect a mechanical section and the like of the magnetic disk. Therefore, it can be said that this problem is peculiar to the SSD device.
As a structure for cooling the controller in the SSD device, it is conceivable to interpose a heat radiation sheet between the controller and a bottom housing. Even if the heat radiation sheet is interposed between the controller and the bottom housing, the entire heat generated from the controller is not always transmitted to the bottom housing. However, a part of the heat is transmitted to the bottom housing via the heat radiation sheet. Consequently, a heat quantity transmitted to the NANDs is reduced to maintain the temperature of the NAND not to exceed the operation guarantee temperature.
According to the improvement of the performance of the SSD device and the computer mounted with the SSD, a heat value during actual operation of the controller tends to increase. For example, a data transfer rate of the SSD in the past is 100 MByte/sec for readout and 40 MByte/sec for writing. Currently, the data transfer rate is improved to 240 MByte/sec for readout and 200 MByte/sec for writing. Therefore, even if the heat radiation sheet is arranged between the controller and the bottom housing, it is difficult to sufficiently radiate the heat generated from the controller. However, an SSD device that takes measures against this problem is not realized.